It is becoming more common place for agricultural vehicles to be driven and controlled by hydraulic systems. These hydraulic systems include a hydraulic pump which is rotated by a suitable prime mover generally at a constant or governed speed. The fluid output from the pump is then controlled by a pump lever which actuates a control plate within the pump from a neutral position at which no fluid is generated to a forward position in which fluid is pumped in a first direction and to a reverse position in which fluid is pumped in the opposed direction. The pump lever does of course move from a position at the neutral location initially to a position of minimum fluid pumping and gradually therefrom to a position of maximum fluid pumping.
Generally the pump itself includes a neutral locator which automatically moves the pump lever to the neutral position and holds it at the neutral position in the absence of a actuating force on the pump lever from a control linkage.
When the pump lever is in the forward or reverse positions a significant force is applied to the pump lever in a direction depending on the pump operating pressures. It is necessary therefore to provide a holding force through the control linkage which holds the pump lever at the required set position.
The holding force requires a frictional resistance to movement in the control linkage which is sufficient to overcome the restoring force generated by the pump. If this holding force is applied to the control linkage at all times then the operator must apply to the control lever a force greater than the holding force to actuate movement of the control lever to the required position. As the operator of the system may be required to adjust the postion of the control lever on a regular basis, this increased force necessary can become tiring and uncomfortable leading possibly to dissatisfaction in the operator and even to operator error.
Furthermore, the provision of a constant frictional force resisting the restoring force from the pump can interfere with the automatic neutral locating device of the pump.
While the description of the present application relates mainly to the control system for a hydrostatic transmission of an agricultural vehicle, the control system is not so limited and can be used for other hydraulic pump control systems.